In addition to using differing frequency spectrums for communication, in code division multiple access (CDMA) communication systems, user communications are separated by the codes used to transmit the communications. In the proposed third generation partnership project (3GPP) wideband code division multiple access (W-CDMA) communication system, the user codes comprise a channelization code and a cell specific scrambling code. Similar arrangements of user or service specific channelization (also called spreading) codes are used in other standards.
In slotted CDMA communication systems, the communications are also separated by time. Each communication for transmission is assigned one or more codes in one or more time slots. In addition in a time division duplex slotted system, uplink and downlink transmissions are separated by assigned time slot.
In slotted CDMA systems, the code/slot assignment is extremely important to the performance of the system. Many factors can degrade the performance of a particular user code/slot assignment, such as interference between neighboring cells, external interferers and interference between users within a cell.
To further enhance the capacity of slotted CDMA system, the use of multiple antenna in cells has grown in interest. These techniques have generally fallen into two categories, fixed and adaptive beamforming. Typically, each beam is generated in such a way that the radiated communication energy is concentrated in a beam and outside of that beam the radiated energy is low. A user located within the beam can easily detect communications transmitted in the beam and users outside the beam are less interfered by signals sent to users in the beam.
In fixed beamforming techniques, the antennas of a cell's base station are configured to transmit communications in multiple fixed beams. These beams are stationary. In adaptive beamforming, beams are formed by adaptive antenna arrays and can be changed. The adaptive beamforming allows for the beams to be moved as the cell loadings change or as users move within a cell.
Beamforming allows for better distinction between individual system users. Not only are the users in a cell distinguished by a time slot and cell, they can also be distinguished by beam. Provided sufficient isolation is achieved, the same codes can be used for different users, thus increasing the capacity of the cell. To illustrate, a user in a first beam may have the same code and time slot assignment as a user in a second beam. These user transmissions are separated by their respective beams.
Although beams allow for additional distinction between users, assignment issues still exist. Individuals within the same beam and bordering on beams can interfere with one another. Users at the peripheries of cells may be located within beams of neighboring cells and, accordingly, interfere with each other.
Accordingly, it is desirable to have efficient time slot and beam assignment schemes in such systems.